Vehicular navigation systems in the known art are provided with a function for displaying a roadmap of the area where the vehicle is currently located, a function for accurately detecting the vehicle position through map matching, a function for calculating a recommended route from a point of departure to a destination and the like. In these vehicular navigation systems in the prior art, roadmap display data, data for map matching and data for route search are separately stored in a CD ROM in order to maintain compatibility with existing software programs and also to improve the processing speed.
The roadmap display data include widest range map data for displaying large areas at the smallest scale, most detailed map data for displaying small areas in detail at the largest scale and a plurality of sets of map data at different scales between that of the widest range map data and that of the most detailed map data. For instance, the widest range map data may be referred to as level 4 data, the most detailed map data may be referred to as level 1 data and the sets of data at scales between those of the level 4 data and the level 1 data may be referred to as level 3 data and level 2 data.
FIG. 23 illustrates roadmaps corresponding to the roadmap display data that are stored in memory as the level 4, level 3 and level 2 roadmap display data as explained above. In the CD ROM, the level 4 roadmap display data, the level 3 roadmap display data and the level 2 roadmap display data are stored separately. FIG. 23 shows a roadmap corresponding to one of the meshes, i.e., the mesh M4, at level 4, and in the mesh M4, a road D1, a road D2 that is connected to an intersection n3 of the road D1 and two roads D3 and D4 that are connected at intersections n1 and n2 at the two ends of the road D1 respectively are present. The hatched small area m3 which is achieved by dividing the one mesh M4 at level 4 into sixteen equal portions constitutes one mesh M3 at level 3 and, as shown in FIG. 23, only portions of the roads D1 and D2 are present in the mesh M3. The hatched small area m2, which is achieved by dividing the mesh M3 at level 3 into sixteen equal portions constitutes one mesh M2 at level 2 and, as shown in FIG. 23, portions of the roads D1 and D2 and roads D5 and D6, which are connected to the road D1 are present in the mesh M2.
In a navigation system with a recommended route search function, route search data are provided as well as the roadmap data described above. Since information on the road forms are not required in route search, the route search data include data on a given node (referred to as the current node in this specification) and nodes connected to the node (referred to as adjacent nodes in this specification). The node data include positional coordinates of the nodes.
For instance, two sets of route search data that correspond to the level 4 roadmap data and the level 2 roadmap data may be provided so that the vicinity of the point of departure and the vicinity of the destination are searched at level 2 in route search and so that the other areas are searched at level 4, to reduce the length of route search time.
In the prior art explained above, it is required that nodes at a high order level and at a low order level be made to correspond with each other with a high degree of efficiency requiring only a small memory capacity and with a high degree of reliability.